Utp14 Recruits and Activates the RNA Helicase Dhr1 To Undock U3 snoRNA from the Preribosome

In eukaryotic ribosome biogenesis, U3 snoRNA base pairs with the pre-rRNA to promote its processing. However, U3 must be removed to allow folding of the central pseudoknot, a key feature of the small subunit. Previously, we showed that the DEAH/RHA RNA helicase Dhr1 dislodges U3 from the pre-rRNA.DHR1 can be linked toUTP14 , encoding an essential protein of the preribosome, through genetic interactions with the rRNA methyltransferase Bud23. Here, we report that Utp14 regulates Dhr1. Mutations within a discrete region of Utp14 reduced interaction with Dhr1 that correlated with reduced function of Utp14. These mutants accumulated Dhr1 and U3 in a pre-40S particle, mimicking a helicase-inactive Dhr1 mutant. This similarity in the phenotypes led us to propose that Utp14 activates Dhr1. Indeed, Utp14 formed a complex with Dhr1 and stimulated its unwinding activityin vitro . Moreover, theutp14 mutants that mimicked a catalytically inactivedhr1 mutantin vivo showed reduced stimulation of unwinding activityin vitro . Dhr1 binding to the preribosome was substantially reduced only when both Utp14 and Bud23 were depleted. Thus, Utp14 is bifunctional; together with Bud23, it is needed for stable interaction of Dhr1 with the preribosome, and Utp14 activates Dhr1 to dislodge U3.